Oxygen production process

ABSTRACT

The invention relates to a process for the production of oxygen using novel membranes, formed from perovskitic or multi-phase structures, with a chemically active coating which demonstrate exceptionally high rates of fluid flux. The process uses membranes that are conductors of oxygen ions and electrons, which are substantially stable in air over the temperature range of 25° C. to the operating temperature of the membrane.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.08/311,295, filed on Sep. 23, 1994, now abandoned and of applicationSer. No. 08/394,925, filed Feb. 24, 1995, now U.S. Pat. No. 5,591,315.Application Ser. No. 08/394,925 is is a continuation of application Ser.No. 08/228,793 filed Apr. 15, 1994, now abandoned, which was adivisional of application Ser. No. 07/618,792 filed Nov. 27, 1990 andissued as U.S. Pat. No. 5,306,411. Application Ser. No. 07/618,792 is acontinuation-in-part of U.S. patent application Ser. No. 07/457,327filed on Dec. 27, 1989, now abandoned; Ser. No. 07/457,340 filed on Dec.27, 1989, now abandoned, which is a continuation-in-part of U.S. patentapplication Ser. No. 07/025,511 filed on Mar. 13, 1987 and issued asU.S. Pat. No. 4,933,054 on Jun. 12, 1990; Ser. No. 07/457,384 filed onDec. 27, 1989, now abandoned; and Ser. No. 07/510,296 filed on Apr. 16,1990, now abandoned which is a continuation-in-part of U.S. patentapplication Ser. No. 07/357,317 filed on May 25, 1989, now abandoned,which are hereby fully incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

Applicants have discovered solid state compositions with enhancedproduct flux, and processes which exploit this property. Membranes,formed from perovskitic or multi-phase structures, with a chemicallyactive coating have demonstrated exceptionally high rates of fluid flux.One application is the production of oxygen. The membranes areconductors of oxygen ions and electrons, and are substantially stable inair over the temperature range of 25° C. to the operating temperature ofthe membrane.

BACKGROUND OF THE INVENTION

Solid state membranes formed from ion conducting materials are beginningto show promise for use in commercial processes for separating,purifying and converting industrial fluids, notably for oxygenseparation and purification. Envisioned applications range from smallscale oxygen pumps for medical use to large gas generation andpurification plants. Conversion processes are numerous, and includecatalytic oxidation, catalytic reduction, thermal treating,distillation, extraction, and the like. Fluid separation technologyencompasses two distinctly different membrane materials, solidelectrolytes and mixed conductors. Membranes formed from mixedconductors are preferred over solid electrolytes in processes for fluidseparations because mixed conductors conduct ions of the desired productfluid as well as electrons, and can be operated without externalcircuitry such as electrodes, interconnects and power-supplies. Incontrast, solid electrolytes conduct only product fluid ions, andexternal circuitry is needed to maintain the flow of electrons tomaintain the membrane ionization/deionization process. Such circuitrycan add to unit cost, as well as complicate cell geometry.

Membranes formed from solid electrolytes and mixed conducting oxides canbe designed to be selective towards specified product fluids, such asoxygen, nitrogen, argon, and the like, and can transport product fluidions through dynamically formed anion vacancies in the solid latticewhen operated at elevated temperatures, typically above about 500° C.Examples of solid electrolytes include yttria-stabilized zirconia (YSZ)and bismuth oxide for oxygen separation. Examples of mixed conductorsinclude titania-doped YSZ, praseodymia-modified YSZ, and, moreimportantly, various mixed metal oxides some of which possess theperovskite structure. Japanese Patent Application No. 61-21717 disclosesmembranes formed from multicomponent metallic oxides having theperovskite structure represented by the formulaLa_(1−x)Sr_(x)Co_(1−y)Fe_(y)O_(3−d) wherein x ranges from 0.1 to 1.0, yranges from 0.05 to 1.0 and d ranges from 0.5 to 0. Some other pertinentperovskite structures have been described in copending application Ser.No. 08/311,295, owned by the Assignee of record herein. The subjectmatter of that application is incorporated herein by reference.

Membranes formed from mixed conducting oxides which are operated atelevated temperatures can be used to selectively separate product fluidsfrom a feedstock when a difference in product fluid partial pressuresexists on opposite sides of the membrane. For example, oxygen transportoccurs as molecular oxygen is dissociated into oxygen ions, which ionsmigrate to the low oxygen partial pressure side of the membrane wherethe ions recombine to form oxygen molecules, or react with a reactivefluid, and electrons migrate through the membrane in a directionopposite the oxygen ions to conserve charge.

The rate at which product fluid ions permeate through a membrane ismainly controlled by three factors. They are (a) the kinetic rate of thefeed side interfacial product fluid ion exchange, i.e., the rate atwhich product fluid molecules in the feed are converted to mobile ionsat the surface of the feed side of the membrane; (b) the diffusion ratesof product fluid ions and electrons within the membrane; and (c) thekinetic rate of the permeate side interfacial product fluid exchange,i.e., the rate at which product fluid ions in the membrane are convertedback to product fluid molecules and released on the permeate side of themembrane, or react with a reactive fluid, such as hydrogen, methane,carbon monoxide, C₁-C₅ saturated and unsaturated hydrocarbons, ammonia,and the like.

U.S. Pat. No. 5,240,480 to Thorogood, et al, incorporated herein byreference, addressed the kinetic rate of the feed side interfacial gasexchange by controlling the pore size of the porous structure supportinga non-porous dense layer. Numerous references, such as U.S. Pat. No.4,330,633 to Yoshisato et al, Japanese Kokai No. 56[1981]-92,103 toYamaji, et al, and the article by Teraoka and coworkers, Chem. Letters,The Chem. Soc. of Japan, pp. 503-506 (1988) describe materials withenhanced ionic and electronic conductive properties.

U.S. Pat. Nos. 4,791,079 and 4,827,07 to Hazbun, incorporated herein byreference, addressed the kinetic rate of the permeate side interfacialgas exchange by utilizing a two-layer membrane in which one layer was animpervious mixed ion and electron conducting ceramic associated with aporous layer containing a selective hydrocarbon oxidation catalyst.

Typical of metal oxide membrane references is Japanese PatentApplication 61-21717, described above. When an oxygen-containing gaseousmixture at a high oxygen partial pressure is applied to one side of amembrane having a dense layer formed from the enumerated oxide, oxygenwill adsorb and dissociate on the membrane surface, become ionized anddiffuse through the solid and deionize, associate and desorb as anoxygen gas stream at a lower oxygen partial pressure on the other sideof the membrane.

The necessary circuit of electrons to supply thisionization/deionization process is maintained internally in the oxidevia its electronic conductivity. This type of separation process isdescribed as particularly suitable for separating oxygen from a gasstream containing a relatively high partial pressure of oxygen, i.e.,greater than or equal to 0.2 atm. Multicomponent metallic oxides whichdemonstrate both oxygen ionic conductivity and electronic conductivitytypically demonstrate an oxygen ionic conductivity ranging from 0.01ohm⁻¹cm⁻¹ to 100 ohm⁻¹cm⁻¹ and an electronic conductivity ranging fromabout 1 ohm⁻¹cm⁻¹ to 100 ohm⁻¹cm⁻¹ under operating conditions.

Some multicomponent metallic oxides are primarily or solely oxygen ionicconductors at elevated temperatures. An example is(Y₂O₃)_(0.1)(Zr₂O₃)_(0.9) which has an oxygen ionic conductivity ofabout 0.06 ohm⁻¹ cm⁻¹ at 1000° C. and an ionic transport number (theratio of the ionic conductivity to the total conductivity) close to 1.European Patent Application EP 0399833A1 describes a membrane formedfrom a composite of this oxide with a separate electronically conductingphase, such as platinum or another noble metal. The electronicconducting phase will provide the return supply of electrons through thestructure allowing oxygen to be ionically conducted through thecomposite membrane under a partial pressure gradient driving force.

Another category of multicomponent metallic oxides exhibit primarily orsolely electronic conductivity at elevated temperatures and their ionictransport numbers are close to zero. An example is Pr_(x)In_(y)O_(z)which is described in European Patent Application EP 0,399,833 A1. Suchmaterials may be used in a composite membrane with a separate oxygenionic conducting phase such as a stabilized ZrO₂. A membrane constructedfrom a composite of this type may also be used to separate oxygen froman oxygen-containing stream, such as air, by applying an oxygen partialpressure gradient as the driving force. Typically, the multicomponentoxide electronic conductor is placed in intimate contact with an oxygenionic conductor.

Organic polymeric membranes may also be used for fluid separation.However, membranes formed from mixed conducting oxides offersubstantially superior selectivity for such key products as oxygen whencompared to polymeric membranes. The value of such improved selectivitymust be weighed against the higher costs associated with building andoperating plants employing membranes formed from mixed conducting oxideswhich plants require heat exchangers, high temperature seals and othercostly equipment. Typical prior art membranes formed from mixedconducting oxides do not exhibit sufficient permeance (defined as aratio of permeability to thickness) to justify their use in commercialfluid separation applications.

Oxygen permeance through solid state membranes is known to increaseproportionally with decreasing membrane thickness, and mechanicallystable, relatively thin membrane structures have been widely studied.

A second article by Teraoka et al, Jour. Ceram. Soc. Japan.International Ed, Vol 97, pp. 458-462, (1989) and J. Ceram. Soc. Japan,International Ed, Vol 97, pp. 523-529, (1989), for example, describessolid state gas separation membranes formed by depositing a dense,nonporous mixed conducting oxide layer, referred to as “the denselayer”, onto a porous mixed conducting support. The relatively thickporous mixed conducting support provides mechanical stability for thethin, relatively fragile dense, nonporous mixed conducting layer.Structural failures due to thermo-mechanical stresses experienced by themembranes during fabrication and use were substantially minimized due tothe chemical compatibility of the respective membrane layers. Based uponconsiderations limited to dense layer thickness, Teraoka and coworkersexpected the oxygen flux to increase by a factor of 10 for a membranehaving a mixed conducting porous layer and a thin mixed conducting denselayer compared to a standard single layered dense, sintered mixedconducting disk. However, they obtained an increase of less than afactor of two.

Perovskitic structures include true perovskites that have a threedimensional cubic array of small diameter metal ion octahedra, as wellas structures that incorporate a perovskite-like layers or layer, i.e.,a two dimensional array of small diameter metal ion octahedra arrangedin a two dimensional square array. These perovskite-like arrays arecharge stabilized by larger diameter metal ions, or other chargedlayers. Examples of perovskitic structures include cubic perovskites,brownmillerites, Aurivillius phases, and the like. A description of therelation between perovskites and some of the various perovskitic phasesis presented in L. Katz and R. Ward, Inorg. Chem. 3, 205-211, (1964),incorporated herein by reference.

These layered structures can accommodate vacancies of oxygen ions, andthe ordering of these vacancies can lead to structural variations, suchas the brownmillerite phase. Brownmillerites are perovskites that haveone sixth of the oxygen ions missing with the resulting oxygen ionvacancies ordered into continuous lines within the crystal. An exampleis SrFeO_(3−x), as described by S. Shin, M. Yonemura, and H. Ikawa inMater Res Bull 13, 1017-1021 (1978). Under conditions where x=0, thestructure is a regular, cubic perovskite structure. As conditions oftemperature and pressure are varied so that x increases, the oxygenvacancies that are introduced are at first randomly scattered throughoutthe crystal, or “disordered”. However, as x approaches 0.5 the vacanciescan become “ordered”, i.e., the vacancies form a regular patternthroughout the crystal. When exactly one sixth of the oxygen ions areabsent (X=0.5) and the resulting vacancies are “ordered”, the phase iscalled a brownmillerite.

Aurivillius phases, sometimes called lamellar perovskites, consist oflayers of perovskite wherein the larger diameter metal cations have, inpart or in toto, been replaced by layers of another oxide, commonly(Bi₂O₂)²⁺, as described in Catalysis Letters 16, p 203-210 (1992) by J.Barrault, C. Grosset, M. Dion, M. Ganne and M. Tournoux. Their generalformula is [Bi₂O₂][A_(n−1)B_(n)O_(3n+1)], where “A” designates thelarger diameter metal ions, and “B” designates the smaller diametermetal ions. Wide latitude of substitution is possible for the A and Bmetals in the perovskite layer, and for Bi in the interleaving layers ofBi₂O₂, as described by A. Castro, P. Millan, M. J. Martinez-Lope and J.B. Torrance in Solid State Ionics 63-65, p 897-901 (1993), incorporatedherein by reference. The so-called superconductors, such asYBa₂Cu₃O_(7−x), are also perovskitic structures, with another type ofordered vacancies, as described in W. Carrillo-Cabrera, H-D Wiemhoferand W. Gopel, Solid State Ionics, 32/33, p 1172-1178 (1989).

Researchers are continuing their search for solid state conductivemembranes which exhibit superior flux without sacrificing mechanical andphysical compatibility of the composite membrane.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to novel mixed conductor membranes whichare capable of separating industrial fluid streams. The membranes have achemically active coating and a structure and composition that form asubstantially perovskitic structure, substantially stable in air overthe temperature range of 25° C. to the operating temperature of themembrane, such that enhanced flux is observed compared to prior artsolid state membranes. The upper range of membrane operating temperaturewould be about 400° C. for partial oxidation of C₂-C₄ hydrocarbons andoxygen production processes; about 700° C. for selected partialoxidation and oxygen production processes, as well as processes for theremoval of oxygen from industrial gases and industrial fluids; and about850° C. for previously mentioned processes under certain circumstances,as well as for partial oxidation of methane and natural gases.

While membranes are known which comprise a mixed conducting oxide layer,the fluid impermeable membranes of the present invention have acomposition that forms a substantially perovskitic structure. Suchstructures exhibit enhanced flux, particularly of oxygen. A porouscoating of metal or metal oxide increases the kinetic rate of the feedside interfacial fluid exchange, the kinetic rate of the permeate sideinterfacial fluid exchange, or both. Membranes fabricated from suchmaterial and in such manner display increased flux.

The fluid impermeable membranes according to the invention are formedfrom a mixture of at least two different metal oxides wherein themulticomponent metallic oxide form a perovskitic structure whichdemonstrates electron conductivity as well as product fluid ionconductivity at temperatures greater than about 400° C. These materialsare commonly referred to as mixed conducting oxides.

Suitable mixed conducting oxides are represented by the structure

[A_(1−x)A′_(x)]BO_(3−δ)

or

(Bi_(2−y)A_(y)O_(2−δ′))(A_(1−x)A′_(x))_(n−1)B_(n)O_(3n+1−δ″))

wherein A is chosen from the group consisting of Ca, Sr, Ba, Bi, Pb, K,Sb, Te, Na and mixtures thereof; A′ is chosen from the group consistingof La, Y, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Th, U,and mixtures thereof; B is chosen from the group consisting of Fe, Mg,Cr, V, Ti, Ni, Ta, Mn, Co, V, Cu, and mixtures thereof; x is not greaterthan 0.9, preferably not greater than about 0.06, more preferably notgreater than 0.4, most preferably not greater than 0.25; y is an integerfrom 0 to 2; n is an integer from 1 to 7; and δ, δ′ and δ″ aredetermined by the valence of the metals.

The mixed conducting oxides are formed into fluid impermeable membranes.At least one surface of the fluid impermeable membrane is coated with aporous layer of metal or metal oxide. The coating acts as a chemicallyactive site which enhances the kinetic rate of the interfacial fluidexchange at the fluid impermeable membrane surface.

The current invention is directed towards a solid state membrane,comprising a structure selected from the group consisting ofsubstantially perovskitic material, an intimate, gas-impervious,multi-phase mixture of an electronically-conductive phase and an oxygenionconductive phase, and combinations thereof; and a porous coatingselected from the group consisting of metals, metal oxides andcombinations thereof.

The current invention is also directed towards the use of one or moremembranes formed from the coated mixed conductors described. Suitableuses of such membranes include processes for the partial oxidation ofC₁-C₄ hydrocarbons, and oxygen separation, production and removal fromoxygen-containing fluids, particularly air, or air diluted with otherfluids.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates perovskitic membranes with a chemicallyactive coating, and processes employing such membranes. One such processis separating oxygen from oxygen-containing feeds at elevatedtemperatures. The membranes are conductors of product fluid ions andelectrons, and are of a composition that forms a substantiallyperovskitic structure. Specific compositions stabilize a perovskiticstructure in the mixed conducting fluid impermeable membrane. Membranesfabricated from such material display increased flux. More particularly,a mixed conductor membrane wherein the fluid impermeable membrane hasthe composition

(A_(1−x)A′_(x))BO_(3−δ)  Equation 1

or

(Bi_(2−y)A_(y)O_(2−δ′))(A_(1−x)A′_(x))_(n−1)B_(n)O_(3n+1−δ″))  Equation2

wherein A is chosen from the group consisting of Ca, Sr, Ba, Bi, Pb, K,Sb, Te, Na and mixtures thereof; A′ is chosen from the group consistingof La, Y, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Th, U,and mixtures thereof; B is chosen from the group consisting of Fe, Mg,Cr, V, Ti, Ni, Ta, Mn, Co, V, Cu, and mixtures thereof; x is not greaterthan 0.9; y is an integer from 0 to 2; n ia an integer from 1 to 7; andδ, δ′, and δ″ are determined by the valence of the metals; wherein theperovskitic phase is substantially stable in air over the temperaturerange of 25-950° C.; and coated with a porous layer of metal or metaloxide selected from the group consisting of platinum, silver, palladium,lead, cobalt, nickel, copper, bismuth, samarium, indium, tin,praseodymium, their oxides, and combinations of the same, has been shownto exhibit unexpectedly high fluid transport fluxes, particularlytransport fluxes of oxygen.

Applicants' discovery can be more fully understood by developing anunderstanding of the mechanism by which product fluids are ionicallytransported through the mixed conducting oxide membrane. Typical productfluids include industrial gases, such as oxygen, nitrogen, argon,hydrogen, helium, neon, air, carbon monoxide, carbon dioxide, synthesisgases (mixtures of CO and H₂), and the like. The product fluid fluxobserved by conventional mixed conductor membranes is controlled bysurface kinetic limitations and bulk diffusion limitations. Surfacekinetic limitations are constraints to product fluid flux caused by oneor more of the many steps involved in converting a feed fluid moleculeon the feed side of the mixed conductor membrane into mobile ions andconverting the ions back to product fluid molecules, or reacting theions with a reactant fluid molecule, on the permeate side of the mixedconductor membrane. Bulk diffusion limitations are constraints on fluidflux relating to the diffusivity of product fluid ions through the fluidimpermeable membrane material.

Membranes composed substantially of perovskitic phase materials exhibithigh overall product fluid flux. However, the perovskitic phase is notformed in all mixed conducting oxide materials or, if formed, is notstable over the required range of fabricating and operating conditions.For instance, membranes formed from hexagonal phase materials exhibitlittle, if any, oxygen flux. To produce an effective oxygen membrane,therefore, the membrane composition must maintain a substantially highfraction of stable perovskitic phase in the membrane at operatingconditions.

To complement the enhanced bulk diffusion rate through the fluidimpermeable membrane, the kinetic rates of interfacial fluid exchangemust be examined. If the rate at which the feed fluid is converted tomobile ions on the feed side of the fluid impermeable membrane, or therate at which the mobile ions in the fluid impermeable membrane areconverted back to product fluid molecules, is slower than the fluidimpermeable membrane bulk diffusion rate, the overall product permeationrate will be limited to the slowest of the three processes.

Applicants have discovered a novel composition which stabilizes theperovskitic phase in fluid impermeable membranes with compositionspreviously unable to sustain a stable perovskitic phase over the rangefrom ambient temperature and pressure in air to the conditions used forproduct fluid separation. In particular, specific compositions stabilizea substantially cubic perovskite layer in a substantially perovskiticstructure. Porous coatings on one or more surfaces of the cubicperovskite material increases the rate of fluid adsorption, ionization,recombination or desorption, as well as increasing the overall fluidflux rate.

The present invention provides coated membranes, and permits thefabrication of mixed conductor oxide structures that are substantiallyperovskitic phase. Membranes made from such material and in such mannerexhibit relatively high overall bulk diffusion rates.

The claimed membranes comprise the composition described in Equation 1and Equation 2, having no connected through porosity, a substantiallystable perovskitic structure in air at 25-950° C., coated with a porouslayer of metal, metal oxides, or mixtures of metals and metal oxides,and the capability of conducting electrons and product fluid ions atoperating temperatures.

Multicomponent metallic oxides suitable for practicing the presentinvention are referred to as “mixed” conducting oxides because suchmulticomponent metallic oxides conduct electrons as well as productfluid ions at elevated temperatures. Suitable mixed conducting oxidesare represented by the compositions of Equation 1 and 2, which yield asubstantially stable cubic perovskite structure in air at 25° C. to theoperating point of material. Materials described in the prior artexhibit significantly lower product fluid fluxes.

The thickness of the material can be varied to ensure sufficientmechanical strength of the membrane. As discussed previously, thinnermembranes increase the overall bulk diffusion rate for a given membranematerial. To exploit this phenomena, thinner membranes may be supportedby one or more porous supports. The minimum thickness of unsupportedmixed conductor membranes of Applicants' invention is about 0.01 mm,preferably about 0.05 mm, most preferably about 0.1 mm. The maximumthickness of unsupported mixed conductor membranes of Applicants'invention is about 10 mm, preferably about 2 mm, most preferably about 1mm.

The minimum thickness of supported mixed conductor membranes ofApplicants' invention is about 0.0005 mm, preferably about 0.001 mm,most preferably about 0.01 mm. The maximum thickness of supported mixedconductor membranes of Applicants' invention is about 2 mm, preferablyabout 1 mm, most preferably about 0.1 mm.

In addition to the increased product fluid flux, the membranes of thepresent invention exhibit stability over a temperature range from 25° C.to the operating temperature of the membrane and an oxygen partialpressure range from 1 to about 1×10⁻⁶ atmosphere (absolute) withoutundergoing phase transitions. Substantially stable perovskiticstructures include all structures with no less than 90% perovskiticphase material, preferably no less than 95% perovskitic phase material,and most preferably no less than 98% perovskitic phase material, whichdo not exhibit permanent phase transitions over a temperature range from25° C. to 950° C. and an oxygen partial pressure range from 1 to about1×10⁻⁶ atmosphere (absolute).

Applicants' invention also includes chemically-active coated membranesof an intimate, fluid-impervious, multi-phase mixture of anyelectronically-conducting material with any product fluid ion-conductingmaterial and/or a gas impervious “single phase” mixed metal oxide havinga perovskite structure and having both electron-conductive and productfluid ion-conductive properties. The phrase “fluid-impervious” isdefined herein to mean “substantially fluid-impervious or gas-tight” inthat the mixture does not permit a substantial amount of fluid to passthrough the membrane (i.e., the membrane is non-porous, rather thanporous, with respect to the relevant fluids). In some cases, a minordegree of perviousness to fluids might be acceptable or unavoidable,such as when hydrogen gas is present.

The term “mixtures” in relation to the solid multi-component membraneincludes materials comprised of two or more solid phases, andsingle-phase materials in which the atoms of the various elements areintermingled in the same solid phase, such as in yttria-stabilizedzirconia. The phrase “multi-phase mixture” refers to a composition whichcontains two or more solid phases interspersed without forming a singlephase solution.

In other words, the multi-phase mixture is “multiphase”, because theelectronically-conductive material and the product fluid ion-conductivematerial are present as at least two solid phases in the fluidimpervious solid membrane, such that the atoms of the various componentsof the multi-component membrane are, for the most part, not intermingledin the same solid phase.

The multi-phase solid membrane of the present invention differssubstantially from “doped” materials known in the art. A typical dopingprocedure involves adding small amounts of an element, or its oxide(i.e., dopant), to a large amount of a composition (i.e., hostmaterial), such that the atoms of the dopant become permanentlyintermingled with the atoms of the host material during the dopingprocess, whereby the material forms a single phase. The multi-phasesolid membrane of the present invention, on the other hand, comprises aproduct fluid ion conductive material and an electronically conductivematerial that are not present in the dopant/host material relationshipdescribed above, but are present in substantially discrete phases.Hence, the solid membrane of the present invention, rather than being adoped material, may be referred to as a two-phase, dual-conductor,multi-phase, or multi-component membrane.

The multi-phase membrane of the present invention can be distinguishedfrom the doped materials by such routine procedures as electronmicroscopy, X-ray diffraction analysis, X-ray adsorption mapping,electron diffraction analysis, infrared analysis, etc., which can detectdifferences in composition over a multi-phase region of the membrane.

Typically, the product fluid ion-conducting materials or phases aresolid solutions (i.e., solid “electrolytes”) formed between oxidescontaining divalent and trivalent cations such as calcium oxide,scandium oxide, yttrium oxide, lanthanum oxide, etc., with oxidescontaining tetravalent cations such as zirconia, thoria and ceria or theproduct fluid ion-conducting materials or phases comprise a productfluid ion-conductive mixed metal oxide of a perovskite structure. Theirhigher ionic conductivity is believed to be due to the existence ofproduct fluid ion site vacancies. One product fluid ion vacancy occursfor each divalent or each two trivalent cations that are substituted fora tetravalent ion in the lattice. Any of a large number of oxides suchas yttria stabilized zirconia, doped ceria, thoria-based materials, ordoped bismuth oxides may be used. Some of the known solid oxide transfermaterials include Y₂O₃-stabilized ZrO₂, CaO-stabilized ZrO₂,Sc₂O₃-stabilized ZrO₂Y₂O₃-stabilized Bi₂O₃, Y₂O₃-stabilized CeO₂,CaO-stabilized CeO₂, ThO₂, Y₂O₃-stabilized ThO₂, or ThO₂, ZrO₂, Bi₂O₃,CeO₂, or HfO₂ stabilized by addition of any one of the lanthanide oxidesor CaO. Many other oxides are known which have demonstrated productfluid ion-conducting ability which could be used in the multi-phasemixtures, and they are included in the present concept.

Preferred among these solid electrolytes are the Y₂O₃-(yttria) andCaO-(calcia) stabilized ZrO₂ (zirconia) materials. These two solidelectrolytes are characterized by their high ionic conductivity, theirproduct fluid ion conduction over wide ranges of temperature and productfluid pressure, and their relatively low cost.

Applicants have also discovered that since perovskitic structuresexhibit excellent electron-conductive and product fluid ion-conductiveproperties, in multi-phase materials, perovskitic materials may be usedas the electronically-conductive material, the product fluidion-conductive material, or both. The resulting multi-phase mixture canbe coated with a porous, chemically active material to produce a solidstate membrane with enhanced flux.

The present invention can consist of a solid state membrane comprisingan intimate, gas-impervious, multi-phase mixture comprising from about 1to about 75 parts by volume of the electronically-conductive phase andfrom about 25 to about 99 parts by volume of the product ion-conductivephase.

The porous coating comprises metal or metal oxide selected from thegroup consisting of platinum, silver, palladium, lead, cobalt, nickel,copper, bismuth, samarium, indium, tin, praseodymium, their oxides, andcombinations of the same, where the coating exhibits oxygen ionicconductivity less than about 1.0 ohm⁻¹cm⁻¹, preferably less than about0.1 ohm⁻¹cm⁻¹, most preferably less than about 0.01 ohm⁻¹cm⁻¹ underoperating conditions. The porous coating may be applied using standardapplications techniques including, but not limited to spraying, dipping,laminating, pressing, implanting, sputter deposition, chemicaldeposition, and the like.

The membranes of the present invention can be used to recover productfluid, such as oxygen, from a product fluid-containing feed fluid bydelivering the product fluid-containing feed fluid into a firstcompartment which is separated from a second compartment by the subjectmembrane, establishing a positive product fluid partial pressuredifference between the first and second compartments by producing anexcess product fluid partial pressure in the first compartment and/or byproducing a reduced product fluid partial pressure in the secondcompartment; contacting the product fluid-containing feed fluid with themembrane at a temperature greater than about 400° C. to separate theproduct fluid-containing feed into a product fluid-enriched permeatestream and a product fluid-depleted effluent stream.

A difference in product fluid partial pressure between the first andsecond compartments provides the driving force for effecting theseparation when the process temperature is elevated to a sufficienttemperature to cause product fluid in the product fluid-containing feedfluid residing in the first compartment to adsorb onto the first surfaceof the membrane, become ionized via the membrane and to be transportedthrough the fluid impermeable membrane in the ionic form. A productfluid-enriched permeate is collected or reacts in the second compartmentwherein ionic product fluid is converted into the neutral form by therelease of electrons at the second surface of the membrane, in thesecond compartment.

A positive product fluid partial pressure difference between the firstand second compartments can be created by compressing the feed fluid,such as air or other oxygen-containing fluid in an oxygen separationprocess, in the first compartment to a pressure sufficient to recoverthe product fluid-enriched permeate stream at a pressure of greater thanor equal to about one atmosphere. Typical pressures range from about 15psia to about 250 psia and the optimum pressure will vary depending uponthe amount of product fluid in the product fluid-containing feed.Conventional compressors can be utilized to achieve the necessaryproduct fluid partial pressure. Alternately, a positive product fluidpartial pressure difference between the first and second compartmentscan be achieved by evacuating the second compartment to a pressuresufficient to recover the product fluid-enriched permeate. Evacuation ofthe second compartment may be achieved mechanically, using compressors,pumps and the like; chemically, by reacting the product fluid-enrichedpermeate; thermally, by cooling the product fluid-enriched permeate; orby other methods known in the art. Additionally, the present inventionmay utilize an increase of product fluid partial pressure in the firstcompartment while simultaneously reducing the product fluid partialpressure in the second compartment, by the means described above. Therelative pressures may also be varied during operation, as necessary tooptimize product fluid separation, or necessitated by process whichsupply feeds to, or accept product streams from, the two compartments.

Recovery of the product fluid-enriched permeate may be effected bystoring the substantially product fluid-enriched permeate in a suitablecontainer or transferring the same to another process. For oxygenproduction processes, the product fluid-enriched permeate typicallycomprises pure oxygen or high purity oxygen defined as generallycontaining at least about 90 vol % O₂, preferably more than about 95 vol% O₂ and especially more than 99 vol % O₂.

Although oxygen separation and purification is described herein forillustrative purposes, the present invention may be used in similarfashion for separation, purification and reaction of other productfluids including, but not limited to, nitrogen, argon, hydrogen, helium,neon, air, carbon monoxide, carbon dioxide, synthesis gases, and thelike.

The following example is provided to further illustrate Applicants'invention. The example is illustrative and is not intended to limit thescope of the appended claims.

EXAMPLE Example 1

A mixed conductor fluid impermeable membrane of nominal composition(La_(0.2)Sr_(0.8))(Co_(0.1)Fe_(0.7)Cr_(0.2)Mg_(0.01))O_(3−δ) wasprepared in a manner similar to the examples described in U.S. Pat. No.5,061,682, incorporated herein by reference. An amount of 4232.60 gramsof Sr(NO₃)₂, 773.80 grams of La₂O₃ (Alpha, dried at 850° C.), 6927.80grams of Fe(NO₃).9H₂O), 730.50 grams of Co(NO₃)₃.6H₂O, and 64.10 gramsof Mg(NO₃)₂ were added to approximately 30 liters of deionized watercontaining dissolved sucrose.

A portable spray-dryer was used to spray-dry the ceramic precursorsolution described above. A suitable portable spray-dryer is availablefrom Niro Atomizer of Columbia, Md. The spray-dryer includes acentrifugal atomizer capable of speeds up to 40,000 rpm. The atomizersits near the top of a drying chamber that has an inner diameter of 2feet, 7 inches, with a 2-foot cylindrical height and a 60° conicalbottom. The centrifugal atomizer and drying chamber are made fromstainless steel. The drying chamber is coupled to an electric air heaterfor providing drying air to the drying chamber. The drying air is drawnthrough the drying chamber by a blower positioned downstream from thedrying chamber. The spray-dryer includes a cyclone separator thatreceives the drying air and dry product from the bottom of the dryingchamber. The cyclone separator separates the dry product from theexhausted drying air. The bottom of the cyclone separator includes anoutlet that allows the dried particles to gravitate into a verticallyoriented tube furnace capable of maintaining an air temperature of about300°-450° C. The dried particles are pyrolyzed in the tube furnace. Thetube furnace has a height sufficient to provide a residence time for thefreely gravitating particles of about 0.5 to 2.0 seconds. The bottom ofthe tube furnace communicates with a collection chamber where theceramic particles are collected.

The ceramic precursor solution described above was introduced into thespray-dryer chamber at a flow rate of about 1.8 liters per hour. Thecentrifugal atomizer spinning at about 30,000 RPM broke up the precursorsolution into small droplets having a diameter on the order of about20-50 microns. The air flow through the drying chamber and cycloneranged between about 35-40 standard cubic feet per minute. The airentering the drying chamber was preheated to about 375° C. As the smalldroplets were forcefully convected toward the bottom of the dryingchamber, they became fully dehydrated down to a critical state ofdehydration such that their diameter was reduced to about 10.0 micronsor less. The temperature of the drying gas at the bottom of the dryingchamber was approximately 125° C., which ensures substantially all thewater was removed from the particles in the spray-dryer. The driedpowder and drying air were then separated from each other in the cycloneseparator. The separated powder fell due to gravity through the tubefurnace, which was preheated to about 490° C. The particles' residencetime in the furnace ranged from about 0.5-2.0 seconds. The temperaturein the tube furnace initiated the exothermic anionic oxidation-reductionreaction between the nitrate ions and the oxides in the individualparticles. The combustion by-products (CO₂ and water vapor) were passedthrough the system and out the exhaust, while the reacted particlesdropped into the collection jar. About 1000 grams of particles werecollected.

The resulting powders were analyzed, and had the composition(La_(0.19)Sr_(0.8))(Co_(0.1)Fe_(0.69)Cr_(0.2)Mg_(0.01))O_(x). A 190.10 gportion of the resulting powder, 3.88 g polyvinyl butyral resin(Monsanto, St. Louis Mo.), and 160 ml toluene were charged, with 820 gof ZrO₂ media to a jar mill, and milled for approximately 3 hours. Anamount of 20 ml absolute ethanol was added, and the slurry allowed tostand, without milling, overnight. The product was filtered, and theresulting powder was dried and screened to pass though a 60 mesh Tylerscreen. The X-ray diffraction (XRD) of the powder showed that thematerial was 100% cubic perovskite phase.

A 4 g portion of the screened powder was pressed into a 1⅜″ diameterdisk under 28,000 psi applied pressure. The disk was fired in air at105° C. for 15 minutes, the temperature increased to 1300° C. over thecourse of 13 hours and maintained for 1 hour, then cooled to ambienttemperature.

The disk was polished on both sides with 500 grit SiC with isopropanolto a final thickness of 1 mm. An amount of 3.0 wt % Bi₂O₃ (Fluka) wasadded to platinum ink (Englehardt), and the resulting mixture wasdiluted with toluene to form a low viscosity coating fluid. Both sidesof the polished disk were coated with the fluid over an area ofapproximately 2.0 cm². The coated disk was heated in air to 1065° C.over a period of 10.5 hours, then cooled to ambient temperature. Thecoated disk was bonded to a 1 inch outside diameter mullite tube with a⅛″ thick Pyrex ring, and the exposed surface area measured to beapproximately 2 cm².

The mullite tube, disk, and gas handling equipment were placed in athermistatically controlled electric heater. The disk was heated instagnant air to 850° C. as indicated by a thermocouple affixed to themullite tube approximately 1 cm from the tube/disk bond. Air flow at therate of 1.0 l/min was initiated on one side of the disk, and heliumpermeate feed flow at 150 cm³/min started on the other side of the disk.The effluent helium permeate was analyzed using on-line gaschromatography. The permeate was also analyzed for nitrogen, to permitcorrection for any air leakage into the permeate stream.

Oxygen flux of the membrane was calculated using the expression:

 q_(o) ₂ =(q _(P)*(x _(O) _(2P) −0.256*x _(N) _(2P) )*P_(O)/760*273/T_(O))/100

where

q_(O) ₂ =Oxygen flux (cm3/min);

q_(p)=Permeate exhaust flow rate (cm3/min);

x_(O) _(2P) =Oxygen concentration in permeate exhaust (%);

X_(N) _(2P) =Nitrogen concentration in permeate exhaust (%);

PO=Atmospheric pressure (mm Hg, abs.); and

T_(O)=Ambient temperature (degrees K).

Oxygen flux was normalized to correct for membrane disk thicknessvariations using the expression:

q′ _(O) ₂ =q _(O) ₂ *L

where

q′_(O) ₂ =Oxygen flux normalized for thickness (cm3/min-mm);

q_(O) ₂ =Oxygen flux (cm3/min); and

L=Thickness of membrane disk (mm).

Oxygen flux per unit area was calculated by dividing the oxygen fluxnormalized for thickness (q′_(O) ₂ ) by the membrane disk area, measuredin cm².

Operating characteristics of the disk were evaluated for over 50 hours.Test data are supplied in Table 1, below. Ambient temperature (T_(O))was maintained at 293° K, and Po was 741 mm Hg for all data points. Theair feed rate was maintained at 1000 sccm.

The data of Table 1 show the excellent long-term stability of thematerial in air at elevated temperatures, and the high oxygen flux.

TABLE 1 Membrane Permeate Time Temp Permeate analysis q_(O2) q′_(O2)(hours) (Deg. C.) (sccm) (x_(O2P)) (x_(N2P)) (cc/min) (cc/cm2/min 1 850154 0.163 0.017 0.256 0.128 6 850 152 0.159 0.017 0.248 0.124 53 850 1540.087 0.020 0.128 0.064

What is claimed is:
 1. A process for the production of oxygen,comprising contacting an oxygen containing fluid with at least one solidstate membrane, wherein said solid state membrane comprises a structureof substantially perovskitic material and a porous, chemically active,electronically conductive coating selected from the group consisting ofmetals, metal oxides and combinations thereof.
 2. The process of claim1, wherein said perovskitic material has a composition selected from thegroup consisting of (A_(1−x)A′_(x))B O_(3−δ),(Bi_(2−y)A_(y)O_(2−δ′))(A_(1−x)A′_(x))_(n−1)B_(n)O_(3n+1−δ″)) andcombinations thereof, wherein A is selected from the group consisting ofCa, Sr, Ba, Bi, Pb, K, Sb, Te, Na and mixtures thereof; A′ is selectedfrom the group consisting of La, Y, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy,Ho, Er, Tm, Yb, Lu, Th, U, and mixtures thereof; B is selected from thegroup consisting of Fe, Mg, Cr, V, Ti, Ni, Ta, Mn, Co, V, Cu, andmixtures thereof; x is not greater than about 0.9; y is an integer from0 to 2; n is an integer from 1 to 7; and δ, δ′, and δ″ are determined bythe valence of the metals.
 3. The process of claim 1, wherein saidperovskitic material is selected from the group consisting of cubicperovskites, brownmillerites, Aurivillius phases, and combinationsthereof.
 4. The process of claim 1, wherein said perovskitic material isa cubic perovskite.
 5. The process of claim 1, wherein said perovskiticmaterial has a composition (A_(1−x)A′_(x))B O_(3−δ) wherein A isselected from the group consisting of Ca, Sr, Ba, Bi, Pb, K, Sb, Te, Naand mixtures thereof; A′ is selected from the group consisting of La, Y,Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Th, U, andmixtures thereof; B is selected from the group consisting of Fe, Mg, Cr,V, Ti, Ni, Ta, Mn, Co, V, Cu, and mixtures thereof; x is not greaterthan about 0.9; and δ is determined by the valence of the metals.
 6. Theprocess of claim 5, wherein A is selected from the group consisting ofCa, Sr, Ba, Bi, and mixtures thereof.
 7. The process of claim 6, whereinA is Sr.
 8. The process of claim 5, wherein A′ is selected from thegroup consisting of of La, Y, Ce, Pr, Nd, and mixtures thereof.
 9. Theprocess of claim 8, wherein A′ is La.
 10. The process of claim 5,wherein B is selected from the group consisting of Fe, Mg, Cr, V, Ti,Ni, Co, V, and mixtures thereof.
 11. The solid process of claim 6wherein A is Sr, A′ is La, and x is less than about 0.25.
 12. Theprocess of claim 5, wherein x is not greater than about 0.6.
 13. Theprocess of claim 5, wherein x is not greater than about 0.4.
 14. Theprocess of claim 1, wherein said perovskitic material has a composition(Bi_(2−y)A_(y)O_(2−δ′))(A_(1−x)A′_(x))_(n−1)B_(n)O_(3n+1−δ″)), wherein Ais selected from the group consisting of Ca, Sr, Ba, Bi, Pb, K, Sb, Te,Na and mixtures thereof; A′ is selected from the group consisting of La,Y, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Th, U, andmixtures thereof; B is selected from the group consisting of Fe, Mg, Cr,V, Ti, Ni, Ta, Mn, Co, V, Cu, and mixtures thereof; x is not greaterthan about 0.9; y is an integer from 0 to 2; n is an integer from 1 to7; and δ′ and δ″ are determined by the valence of the metals.
 15. Theprocess of claim 1, wherein said perovskitic material is abrownmillerite.
 16. The process of claim 1, wherein said perovskiticmaterial is selected from the group consisting of Aurivillius phases.17. The process of claim 1, wherein said porous coating is selected fromthe group consisting of platinum, silver, palladium, lead, cobalt,nickel, copper, bismuth, samarium, indium, tin, praseodymium, theiroxides, and combinations of the same.
 18. The process of claim 1 whereinsaid electronically-conductive phase comprises silver, gold, platinum,palladium, rhodium, ruthenium, bismuth oxide, a praeseodymium-indiumoxide mixture, a cerium-lanthanum oxide mixture, a niobium-titaniumoxide mixture, or an electron-conductive mixed metal oxide of aperovskite structure, or a mixture thereof and the oxygen ion-conductivephase comprises yttria- or calcia-stabilized zirconia, ceria or bismuthoxide, or an oxygen ion-conductive mixed metal oxide of a perovskitestructure.
 19. A process for the production of oxygen, comprisingcontacting an oxygen containing fluid with at least one solid statemembrane, wherein said solid state membrane comprises a structure of anintimate, gas-impervious, multi-phase mixture comprising from about 1 toabout 75 parts by volume of the electronically-conductive phase and fromabout 25 to about 99 parts by volume of the product ion-conductive phaseand a porous, chemically active, electronically conductive coatingselected from the group consisting of metals, metal oxides andcombinations thereof.
 20. The process of claim 19, wherein said porouscoating is selected from the group consisting of platinum, silver,palladium, lead, cobalt, nickel, copper, bismuth, samarium, indium, tin,praseodymium, their oxides, and combinations of the same.